1. Field of the Invention
The present invention relates to sensitivity controls for camera systems. In particular, the present invention relates to sensitivity controls for charge coupled device imagers.
2. Description of the Prior Art
Charge coupled device, CCD, imagers are known to operate over the silicon response spectral band. These devices suffer the deficiency of limited dynamic range. While the devices have ample sensitivity for daylight operation, many CCD imagers suffer from saturation when the scene intensity levels reach the order of 200 times the minimum detectable signal level for surface channel devices. If the system is to operate throughout daylight hours, an overall light intensity range on the order of 5000:1 may be experienced. As a result, charge coupled device camera systems have been required to use a set filter for a specific light level of operation. If a filter is adequate to permit maximum light intensity in daylight operations, the CCD camera loses its sensitivity to low light level conditions. Conversely, low level filters or no filtering of the CCD device for low light level conditions can be defeated by high intensity light levels which overwhelm the camera system in daylight operation.
The sensitivity of a CCD or any other photodetector may be controlled by either of/or a combination of two methods. These methods are (1) control of the rate of integration of photon generated charge; and (2) control of time of integration of photon generated charge in the detector, i.e., exposure control. Since the quantum efficiency of a present day CCD imager cannot be readily altered without image degradation, control of the integration rate of usable photon generated charge must be achieved by means of controlling the quantity of photons reaching the detector. In order to accomplish this, an obstruction in the optical path is required. Considering all CCD seeker system requirements, it becomes clear that a catadioptric optical imaging system should be used which has a central obscuration. A catadioptric system is a combination reflective and refractive optical system. The use of an iris or other methods which obscure the entrance pupil in a non-uniform manner result in image quality degradation. If an obstruction in the optical path is to be used, it must provide uniform optical restriction across the optical aperture, such as a neutral density filter with controllable density. The second approach to sensitivity control, exposure control, is via control of the integration time period of the imager. This in turn may be achieved by either or a combination of two methods: (1) control of the time interval during which photons are allowed to reach the detector; or (2) control of the time period during which photon generated signal charge is allowed to accumulate.
Frustrated total internal reflection devices are known optical techniques. The basic principle relies on the fact that, when light passes from a high index material to a low index material, a portion of the light wave is transmitted and reflected in accordance with Snell's law. ##EQU1## Snell's law is also described with reference to FIG. 1 which shows the relationship between reflection and refraction at an interface. Light incident along path I strikes the surface at an angle .theta. with respect to a normal, N, to the surface. If the index n in the incident path is greater than the index n', the light will be divided into two components T and R as shown. The reflected light comes out along path R, where the angle of incidence is equal to the angle of reflection. The transmitted light, T, is transmitted as angle .theta.' with respect to a normal to the surface. When the incident angle exceeds the critical angle, .theta..sub.c, EQU .theta..sub.c =Sin.sup.-1 (n'/n) EQ. 2
all of the light is reflected back into the high index material, that is total internal reflection, TIR. Thus, none of the light would be transmitted along path T. However, even though the energy is totally internally reflected, electro-magnetic fields do penetrate the surface and exist in the lower index material for a very short distance. If a third medium with index of refraction equal to the first, is introduced very close to the first, FIG. 2, the TIR can be frustrated to such a degree that essentially all of the light is transmitted.
With reference to FIG. 2, the amount of light transmitted is a function of the distance, D, between the two high index mediums. If D is much less than a wavelength of the incident light, transmission can be very high. If D is greater than a wavelength, the light will be totally reflected. The relation for the percent of light transmitted is: ##EQU2## In this equation T.sigma. and T.pi. are the transmittance for light polarized parallel and perpendicular, respectively, to the interface. ##EQU3## .theta. is the angle of incidence at the interface n is the index of refraction of the prisms
.lambda. is the wavelength of incident light PA1 D is the gap spacing.
This permits a light switch or variable light valve to be built with a device which is controled by spacing D. FIG. 3 shows a typical response curve for an FTIR prism.
The smear noise of a frame transfer CCD device is the result of a charge generated during the transfer period. It is reduced by eliminating photon access to the sensor during that time period. This is done by introducing an optical switch into the optics train that shuts off light during the transfer period. The requirements for a switch to operate in this mode are a turn off time of 15 to 20 microseconds, a turn on time of 300 to 350 microseconds, a duty cycle of 15.4 milliseconds on, 1.3 milliseconds off, and an on-state transmittance greater than or equal to 80% uniform aperture, an off-state transmittance of 0.01%. It can be seen from these requirements that most mechanical and electro-optical devices are eliminated.
Control of the integration time of a CCD camera permits improvement of the sensitivity control for two reasons. The first approach is to control the V.sub..phi.VAH signal to the CCD imager chip. This V.sub..phi.VAH signal controls the photon generated charge integration capabilities of the individual imager cells. The problems with this technique are (1) when the V.sub..phi.VAH control is switched from low to high during the time that video is being read out by the CCD, a transient is coupled into the video which results in a white line across the image and (2) a significant deficiency in dynamic range is encountered by this technique. When the integration period is lowered to 1/8 of a normal field period, image smearing occurs and increases as the integration period is shortened. It remains to be seen whether the former of these problems is one of EMI, electro-magnetic interference, and can be eliminated by careful electronic design. The latter problem can be explained as follows; the field transfer technique used by the CCD requires on the order of 0.9 milliseconds to shift the image-generated charge pattern from the image array to the storage and readout array. During the 0.9 millisecond transfer period, photon generated charge is being generated as the charge is clocked from the image array to the storage array. The charge generated during this transfer period may be considered as noise since it doesn't contribute to the image signal. This noise appears as a smeared image. Assuming that the photon intensity is the same during transfer as during image integration, the signal to noise ratio for this type of noise is on the order of 17 to 1 with 16.67 millisecond field integration time. In this case, the charge generated by the signal is much larger than the noise charge generated during transfer. However, as the signal charge integration time is decreased, the signal to noise ratio decreases and the image starts to smear. It has been noticed that at integration times of the order of 1/8th of the full integration time, the scene started to smear. This corresponds to about 2:1 signal to smear noise ratio.
The net effect of this problem is to impose a limit of approximately 8:1 on the dynamic range. One method to get around this is to increase the field transfer rate. This decreases the transfer period. When this is done, the noise collection time is smaller and hence, the noise signal is less. There are two problems with this approach, first a definite time is required to shift out the image data since the dynamic range may be increased but only to the point that the image integration time approaches the transfer period, and secondly for a surface channel CCD the transfer rate is already at its upper limit and further increases would introduce image degradation.